The invention relates to immersion lithography apparatus and methods, and particularly to apparatus and methods for controlling the immersion liquid between the projection system and the substrate.
A typical lithography apparatus includes a radiation source, a projection optical system and a substrate stage to support and move a substrate to be imaged. A radiation-sensitive material, such as a resist, is coated onto the substrate surface before the substrate is placed on the substrate stage. During operation, radiation energy from the radiation source is used to project an image defined by an imaging element (such as, for example, a mask) through the projection optical system onto the substrate. The projection optical system typically includes a plurality of lenses. The lens or optical element closest to the substrate can be referred to as the last or final optical element.
The projection area during an exposure is typically much smaller than the imaging surface of the substrate. The substrate therefore has to be moved relative to the projection optical system to pattern the entire surface of the substrate. In the semiconductor industry, two types of lithography apparatus are commonly used. With so-called “step and repeat” apparatus, the entire image pattern is projected at once in a single exposure onto a target area of the substrate. After the exposure, the wafer is moved or “stepped” in the X and/or Y direction and a new target area is exposed. This step and repeat process is performed over and over until the entire substrate surface is exposed. With scanning type lithography apparatus, the target area is exposed in a continuous or “scanning” motion. The imaging element is moved in one direction, while the substrate is moved in either the same or the opposite direction during exposure. After each scan, the substrate is then moved in the X and/or Y direction to the next scan target area. This process is repeated until all the desired areas on the substrate have been exposed.
Lithography apparatus are typically used to image or pattern semiconductor wafers and flat panel displays. The term “substrate”, as used herein, is intended to generically mean any work piece that can be patterned, including, but not limited to, semiconductor wafers and flat panel displays.
Immersion lithography systems use a layer of fluid that fills a gap between the final optical element of the projection optical system and the substrate. The fluid enhances the resolution of the system by enabling exposures with a numerical aperture (NA) greater than one, which is the theoretical limit for conventional “dry” lithography. The fluid in the gap permits the exposure with radiation that would otherwise be completely internally reflected at the optical-air interface. With immersion lithography, numerical apertures as high as the index of refraction of the fluid are possible. Immersion also increases the depth of focus for a given NA, which is the tolerable error in the vertical position of the substrate, compared to a conventional dry lithography system. Immersion lithography therefore has the ability to provide greater resolution than can be performed using conventional dry lithography, as the fluid essentially becomes part of the optical system of the lithography apparatus.
One known way of maintaining the immersion fluid in the gap where exposure of the substrate is to occur is with the use of an air curtain. For more information on air curtain type immersion apparatus, see for example U.S. Patent Publication No. 2005/0007569, the disclosure of which is incorporated herein by reference in its entirety.
It is also known to maintain the immersion fluid in the gap between the final optical element and the imaging surface of the substrate by submersing both in a container filled with immersion fluid. See, for example, U.S. Pat. No. 4,509,852, the disclosure of which is incorporated herein by reference in its entirety.
Another known way of maintaining the immersion fluid within the gap of an immersion lithography apparatus is with the use of a confinement member that surrounds the final optical element immediately above the area to be exposed on the substrate. For more information on confinement member type immersion lithography apparatus, see U.S. Pat. No. 7,292,313 and WO 2005/111722, the disclosure of each of which is incorporated herein by reference in its entirety.
In yet another approach, which is a variation of the above-described submersion type apparatus, a large confinement plate is used for submerging the substrate to be imaged in the immersion fluid. For more details on confinement plate type immersion lithography apparatus, see U.S. Patent Publication No. 2007/0279608, the disclosure of which is incorporated herein by reference in its entirety.
During semiconductor wafer fabrication, for example, wafers are typically patterned one after another by the lithography tool. After a wafer has been patterned, it is replaced and the next wafer is exposed. This process is completed over and over, typically as fast as possible, to increase throughput. During a wafer exchange, the just exposed wafer typically has to be moved a relatively long distance from the exposure area to the wafer exchange area. This movement preferably is performed at a relatively high speed. Once the exchange takes place, the new wafer undergoes another relatively long-move to an alignment area. This movement also preferably is performed at a relatively high speed. After alignment, the wafer undergoes yet another long-move back to the exposure area for exposure. This movement also preferably is performed at a relatively high speed. For the sake of simplicity, all of the above-described moves are hereafter generically referred to as “long-moves”.
It also is desirable to move from target area to target area on a substrate during the exposure operation at a relatively high speed. Furthermore, when exposure of a target area begins in a scanning exposure apparatus, it is desirable to accelerate at as high a rate as possible at the beginning of the scanning movement and to scan across the target area during exposure of the target area at as high a speed as possible.
High speed movements are desirable in order to increase the throughput (the rate of production) of the exposure process performed by the lithography apparatus.
High speed moves can be problematic with confinement member type immersion apparatus. In particular, if the speed is too fast, there is a tendency for the immersion fluid to leak out from under the confinement member, leaving a trail of water (or other liquid if a liquid other than water is used as the immersion fluid) behind on the substrate. Liquid that escapes from the liquid confinement member also can scatter inside the exposure apparatus, which can deteriorate components of the exposure apparatus and/or adversely change the environmental conditions (for example, temperature and humidity) within the apparatus.
U.S. Patent Publication No. 2007/0110213 discloses a plate disposed on a barrier member that confines a liquid between the projection system and the substrate. The plate separates the liquid-filled space between the barrier member and the substrate into upper and lower channels, such that a meniscus is formed in each of the channels.